Ubiquinone is an essential component of the mitochondrial respiratory chain. Although present in all organs and tissues of animals, the concentration of ubiquinone is highest in heart and skeletal muscle. Ubiquinone synthesis and localization is usually considered to be limited to the inner mitochondrial membrane. However, a recent report has shown that ubiquinone is also synthesized on the ER of rat liver. In fact, Golgi vesicles and lysosomes, membranes in close communication with the ER, contain more ubiquinone on a protein basis that mitochondria. The function ubiquinone might be serving in these other intracellular compartments is not clear. Since ubiquinone biosynthesis in yeast and other eukaryotes have share the same pathway, the proposed studies take advantage of a class of respiratory defective mutants of Saccharomyces cerevisiae that are deficient in ubiquinone. The mutants fall into nine complementation groups, and will be used to characterize the regulation of ubiquinone biosynthesis and to determine the function of ubiquinone in nonmitochondrial compartments. Yeast mutants defective in 3,4-dihydroxy-5-hexaprenyl benzoate (DHHB) methyltransferase, a regulated enzyme of ubiquinone synthesis, will allow the gene encoding this enzyme to be cloned. The gene will provide the biochemical tools necessary to study the regulation of DHHB methyltransferase activity and its role in the regulation of ubiquinone biosynthesis. DHHB methyltransferase null mutants will be constructed to assess whether a lack of mitochondrial ubiquinone synthesis affects ubiquinone auxotrophic or synthesis in other organelles. The lesions in other ubiquinone auxotrophic mutants will be identified. The yeast mutant strains will be used as vehicles for isolating human cDNA clones, either by homology probing strategies or by functional complementation. Experimental therapies have been reported in which administration of ubiquinone to patients with heart disease, mitochondrial encephalomyopathies, or with Kearns-Sayre indicates that ubiquinone may aid in preventing mevinolin induced muscle weakness, a rare but major side effect of hypocholesterolemic therapy with lovastatin. The mechanisms by which ubiquinone mediates these effects is not clear. The proposed studies should increase our understanding of how ubiquinone functions in these clinical therapies.